Cell membrane structureAQA A-Level Biology: Revision notes
Section 1
Basic structure of all membranes
The cell-surface membrane and the membranes around organelles in eukaryotic cells have the same basic structure. A membrane controls what enters and leaves a cell or organelle and separates compartments with different conditions.
The components are phospholipids, cholesterol, proteins, and carbohydrate chains attached to lipids and proteins (glycolipids and glycoproteins).
Section 2
The phospholipid bilayer
A phospholipid has a hydrophilic head (containing phosphate) and two hydrophobic fatty acid tails. In water, they form a bilayer: heads face outwards towards the water on both sides and tails face inwards, forming a hydrophobic core.
The core is a barrier to ions and polar molecules, which cannot easily dissolve in it. Small non-polar molecules, such as oxygen, can pass through.
Do not say tails are 'water hating molecules'. Say the hydrophobic tails are repelled by water so they face inwards.
Section 3
Proteins and carbohydrates
Proteins are embedded in the bilayer. Some span the whole membrane (channel and carrier proteins) and others lie on one surface. They are scattered unevenly.
Glycoproteins (carbohydrate chain attached to a protein) and glycolipids (carbohydrate chain attached to a lipid) project from the outer surface and are involved in cell recognition.
Section 4
The fluid-mosaic model
In the fluid-mosaic model the phospholipids and many proteins can move within their layer, so the membrane is fluid. The proteins are scattered through the bilayer in a pattern like a mosaic.
Section 5
Cholesterol
Cholesterol may be present in cell membranes. It lies between the phospholipids and restricts the movement of the other molecules in the membrane, making the membrane more stable and less fluid.
Section 6
Required practical 4: membrane permeability
Beetroot cells contain red pigment in the vacuole. If membranes are damaged or more permeable it leaks out.
- Cut equal-sized beetroot discs with a cork borer and rinse them.
- Place one disc in each tube, with a different value of the independent variable (for example temperature or ethanol concentration).
- Keep other variables (volume, time, disc size) the same.
- Remove the discs and measure absorbance with a colorimeter (blue-green filter, zeroed with distilled water).
- Repeat and calculate a mean.
Higher absorbance means more pigment has leaked, so the membrane is more permeable. High temperatures denature proteins and disrupt the bilayer; ethanol dissolves phospholipids.
Always name the control variables: disc size, volume of solution, time and temperature.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Cell membrane structure
- A technician cut equal-sized discs from a beetroot with a cork borer and then rinsed them in distilled water before starting an experiment on membrane permeability.The technician placed other discs in ethanol. Explain why the solution around these discs became redder than the solution around discs in water.2 marks
- Phospholipid molecules have a hydrophilic head containing a phosphate group and two hydrophobic fatty acid tails. A cell-surface membrane has an aqueous solution on both sides: tissue fluid outside the cell and cytoplasm inside it.Explain why sodium ions cannot easily diffuse through the middle of the membrane.2 marks
- The cell-surface membrane of a liver cell contains phospholipids, cholesterol, proteins and short carbohydrate chains. In 1972 Singer and Nicolson proposed the fluid-mosaic model of membrane structure.Describe the arrangement of the phospholipids, proteins and carbohydrates in this membrane.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).